There has been a significant increase in the use of electronic scanners for the preparation of half-tone colour separations from continuous tone colour originals. These electronically-modulated high resolution raster scanners scan the photographic element with a very small spot of high intensity light emitted by various lasers, such as (1) a gas laser, e.g. argon ion at 488 nm, helium-neon at 633 nm or helium-cadmium at 442 nm, (2) a near infrared laser diode emitting in the range 750-1500 nm or (3) a light-emitting diode (LED) emitting in either the visible or the near infrared. The exposing spot of light is scanned rapidly across the photo-sensitive material so that the dwell time on any part of the film is typically from 10.sup.-7 to 10.sup.-6 seconds.
The half-tone pattern is produced by means of electronic dot generation (EDG), whereby a number of image pixels produced by the exposure are combined to form the half-tone dot of the required size. Satisfactory dots can be obtained using medium to high contrast materials processed with rapid access chemistry and it is found unnecessary to use the ultra-high contrast "lith" systems which are essential when dots are produced by the traditional optical screening methods.
The contrast requirements for a rapid access processed material can be fulfilled with a silver halide emulsion of narrow grain size distribution containing a contrast enhancing metal dopant, typically, a Group VIII metal complex.
One problem associated with electronic scanners is the need to image the film with a microsecond or sub-microsecond exposure time. Silver halide photographic materials usually respond optimally to exposure times in the range of 1 to 100 milliseconds, and tend to perform less efficiently under microsecond exposures, showing significant losses in both sensitivity and contrast. This is due to the phenomenon of high intensity reciprocity failure (HIRF). In addition to the reduction of sensitivity and contrast, HIRF can also account for a number of related problems, e.g.:
(1) intermittency effects, which cause multiple superimposed short exposures to have a progressively greater effect as the time interval separating them increases from microseconds, to milliseconds or longer; PA1 (2) latent image progression, whereby the latent image gives a stronger developed image, when the interval between exposure and development is of the order of up to one hour; PA1 (3) unusually high sensitivity to developer conditions, e.g. state of exhaustion of the developer.
It is desirable for a scanner material to have a HIRF response that has been reduced to a low level, or preferably eliminated completely, so that the photographic response is independent of the exposure duration.
The use of Group VIII metals as dopants in photographic silver halide emulsions has been known for many years. The dopants are most advantageously added during the crystal growth stages of emulsion preparation, i.e. during initial precipitation and/or physical ripening of the silver halide crystals. Incorporation of these metal dopants into normal, negative-acting photographic emulsions can produce a number of different photographic effects depending on the nature of the metal dopant. Thus, the Group VIII metal complexes are not all equivalent as far as their effect on photographic silver halide emulsion is concerned.
For example, the incorporation of certain Group VIII metal salts results in an enhancement of contrast together with an overall desensitisation. Rhodium salts have found the greatest utility in this respect, as disclosed, for example, in British Patent Specification No. 775 197 using rhodium trichloride, and British Patent No. 1,535,016 using sodium hexachlororhodate. Similar effects have been produced by incorporation of ruthenium, palladium, osmium and platinum as reported by J. W. Mitchell (Photog. Sci. and Eng. 27 (2) p 81 1983) and Research Disclosure 13452 June 1975.
However, quite different effects are obtained with the incorporation of iridium salts. Improvements in sensitivity to high intensity exposure and the reduction in desensitisation caused by mechanical stress have been reported for iridium doped photographic silver halide emulsions in British Patent Specification Nos. 1 527 435 and 1 410 488 and U.S. Pat. Nos. 4,126,472 and 3,847,621.
Certain advantages have been reported for specific combinations of metal ions, for example, British Patent Specification No. 1 395 923 discloses that a mixture of rhodium and iridium complexes provides high contrast to photographic silver halide emulsions whilst avoiding post-exposure latent image intensification. U.S. Pat. No. 3,790,390 discloses this mixture in combination with certain sensitising dyes providing increased sensitivity to microsecond exposure.
U.S. Pat. Nos. 2,448,060, 3,703,584, 3,980,154, 4,147,542 and 4,173,483 disclose photographic silver halide emulsions containing at least one compound containing a metal belonging to Group VIII of the Periodic Table. However, whilst these patents disclose some examples employing the combination of two compounds of different Group VIII metals, e.g. iridium and rhodium there is no exemplification of the combination of iridium and ruthenium compounds.
It has been found that the combination of particular iridium and ruthenium dopants in photographic silver halide emulsions provides surprising and particularly advantageous properties.